Spent caustic streams resulting from the removal of sulfur from petroleum feed streams produced in refineries and gas plants are highly alkaline, odiferous and toxic, and contain sulfidic, phenolic and naphthenic sodium salts. Environmental concerns and legislation dictate that such spent caustic may no longer be disposed of by direct discharge into the sea.
Refinery and gas plant spent caustic is produced in large quantities of up to 150 gallons per hour. This caustic solution contains sulfides, bisulfides, mercaptides, sulfite, sulfate, thiosulfate, phenols, naphthenates, carboxylates, carbonates, bicarbonates, bisulfites, bisulfates, hydroxides, sodium, sulfonic acids, and many other sulfurous and organic and inorganic species. Neutralization of spent caustic with mineral acids releases odiferous hydrogen sulfide, mercaptans, and other toxic and objectionable species. Discharge of spent caustic to surface waters is largely forbidden because of toxicity and alkalinity. Discharge to sewage treatment is usually not possible because of biotoxicity concerns. In some industrialized countries, spent caustic can be sold for the production of various specialty chemicals. In general, disposal of spent caustic is becoming a major problem and operating expense in oil-producing regions of the world where sulfur is present.
Removal of sulfur compounds from gas streams has been of considerable importance in the past and is even more so today due to environmental considerations. Gas effluent from the combustion of organic materials, such as coal, almost always contain sulfur compounds and sulfur removal processes have focused on removing hydrogen sulfide since it has been considered a significant health hazard and because it is corrosive, particularly when water is present. With increasing emphasis on eliminating or minimizing sulfur discharge to the atmosphere, attention is turning to the removal of other sulfur compounds from gas streams.
Numerous natural gas and petroleum oil wells produce what is referred in the industry as “sour gas” containing hydrogen sulfide, mercaptans, sulfides and disulfides in concentrations that make its use unacceptable. Considerable effort has been expended to find effective and cost-efficient means to remove these objectionable sulfur compounds from natural gas.
The removal of sulfur compounds, and particularly chemically-combined sulfur, such as organosulfur compounds, from feedstreams is highly desirable for environmental reasons and in order to prevent potential catalyst deactivation as well as equipment corrosion. This has become of particular concern to the automotive industry as developing countries try to meet gasoline and diesel fuel needs by using deeper high-sulfur petroleum feed stocks without sweetening processes.
Typically, hydrocarbon products contain various amounts of sulfur compounds in the form of, for example, chemically-combined sulfur, such as inorganically combined sulfur and organically combined sulfur, i.e., organosulfur compounds.
The presence of organosulfur compounds in hydrocarbon streams occurs naturally, and as a result of their introduction during conventional processes for producing and treating hydrocarbon products. As previously noted, if chemically-combined sulfur, such as organosulfur compounds, are not removed from the hydrocarbon stream their presence in the resultant hydrocarbon products, including natural gas, paraffins, olefins and aromatics can cause corrosion of processing equipment and, in the case of gasoline and other fuels, of engine parts. Other deleterious effects occur particularly when water is present.
Various industrial processes utilize an aqueous solution of sodium hydroxide, sometimes referred to herein simply as caustic, to wash reaction products or to remove undesirable compounds as a means of purifying desired reaction products. For instance, caustic is used to wash petroleum products in order to remove undesirable compounds from a feed stock prior to polymerization of propene and butene to obtain high octane gasoline blending components. Such polymerization reactions typically take place under high pressure in the presence of a phosphoric acid catalyst and the hydrocarbon feed stock must be free of sulfur to avoid poisoning the catalyst, basic materials which neutralize the catalyst, and oxygen which deleteriously affects the polymerization reaction. Accordingly, the propene and butene feed stock is washed first with caustic to remove mercaptans and subsequently with an amine solution to remove hydrogen sulfide. Subsequently, the feed stock is washed with water to remove the caustic and amine reaction product. The resulting waste water stream accordingly contains caustic, amines and mercaptans.
In petroleum refining, chemical treatment is used to remove or change the undesirable properties associated with sulfur, nitrogen or oxygen compound contaminants in petroleum feed stocks. The chemical treatment process involves extraction or oxidation, which is also known as sweetening. A prior art extraction process that is used to remove mercaptans from propane/propylene and butane/butylene feed streams is known as the Merox Process. These streams may also undergo treatment with an amine before Merox Process extraction to remove excess hydrogen sulfide that tends to fractionate with propane/propylene and interferes with the Merox process. A caustic prewash removes any remaining trace of hydrogen sulfide prior to the Merox Process extraction. These streams are passed up through the trays of an extraction tower. A caustic solution flowing down the extraction tower absorbs mercaptans. The rich caustic is then regenerated by oxidizing the mercaptans to disulfide in the presence of aqueous Merox Process catalyst and the lean caustic is recirculated to the extraction tower. The disulfide is insoluble in the caustic and can be separated.
Oxidation or “sweetening” is used in the purification of gasoline and distillate fractions. A common oxidation process is also a Merox Process that uses a solid catalyst bed. Air and a minimum amount of the alkaline caustic is injected into the hydrocarbon stream. As the hydrocarbon passes through the Merox Process catalyst bed, sulfur mercaptans are oxidized to disulfide. The disulfide can remain with the gasoline product, since it does not possess the objectionable odor properties of mercaptans. Caustic solutions can also be used to absorb and remove hydrogen sulfide and phenol contaminants from intermediate and final product streams during the refining of petroleum. Aqueous caustic waste streams containing phenols can be recycled by reducing the pH of the aqueous caustic until the phenols become insoluble, thereby allowing physical separation.
Caustic soda and soda ash have been used as an alkaline source in the liquid scrubbing of sulfur dioxide present in gases produced from crude oil-fired steam generators. The use of caustic allows the sulfur dioxide scrubber to run at a lower pH with a higher sulfur dioxide removal capacity when compared to the use of soda ash. Such processes result in a large amount of waste caustic solution which heretofore has been disposed of by flushing into sewers.
As is well known to the prior art, many intermediate and final streams from plants for the processing of petroleum products contain a variety of acid compounds such as hydrogen sulfide, mercaptans, phenols, thiophenols and naphthenic acids that must be removed or reduced in concentration. The compounds containing sulfur must be reduced to concentrations low enough to reduce odor. Aqueous solutions of sodium hydroxide are commonly used in concentrations of between five and fifteen percent by weight to treat petroleum products to obtain the desired reduction in concentration of the undesired components.
Spent caustic soda solutions of the prior art will have different compositions depending upon whether the caustic solution has been used for purification of propane and butane gases or in the purification of petroleum feed stocks obtained from thermal and/or catalytic “cracking” or of “straight-run” hydrocarbons which are obtained by distillation of crude oil at atmospheric pressure. However, in general, the spent caustic soda solutions have pH values ranging from 12.5 to 13.5 and the following compositions expressed in percent by weight: free caustic soda 5.0-7.5; total oils 0.5-2.0; total sulfides 0.1-3.0; cyanides 0.05-0.3; ammonia 0.05-0.4; phenols 0.2-10; lead 2×10−4 to 4×10−4; arsenic 1×10−4 to 5×10−4; copper 5×10−4 to 50×10−4; cadmium 1×10−4 to 5×10−4; and the balance of the composition being water.
Many processes have been developed to treat spent caustics. Most of these processes are expensive with regard to both capital installation and operating costs.
For example, in U.S. Pat. Nos. 5,589,053 and 5,667,668, an electrolysis process for removal of caustic in a hemicellulose/caustic solution is disclosed. The caustic is recovered by electrolysis in a membrane type electrolytic cell utilizing as an electrolyte a mixture of hemicellulose and caustic which is essentially free of lignin. By electrolysis, the concentration of caustic in the anolyte compartment of the cell is decreased and the concentration of caustic in the cathodic compartment of the cell is increased so as to allow recovery of about 60 to about 80% of the caustic contained in the hemicellulose/caustic starting solution.
Mercaptans have been removed in the prior art from caustic scrubber solutions by blending the caustic with oxidation agents, such as tannic acid, and then blowing air through the spent caustic scrubber solutions to oxidize the mercaptans to disulfides. The disulfides are then either skimmed off as an oily layer or further oxidized to thiosulfate or sulfate using other techniques. The prior art spent caustic scrubber solutions are then either neutralized and sewered or the oxidized sulfur compounds are precipitated with precipitating agents, such as iron, and the precipitate which is formed is removed by filtration leaving the caustic solution filtrate clean enough to be returned to the scrubber. Because of the cost of the precipitation option, current prior art practice mostly employs the neutralization and sewering steps. This process requires large amounts of sulfuric acid which greatly increases the amount of sulfate effluent waste which is discharged from the refinery.
Regeneration of a spent caustic solution used to scrub sour gasoline to remove oxidizable sulfur compounds such as mercaptans, hydrogen sulfides, and other compounds is disclosed in U.S. Pat. No. 2,654,706. The spent caustic solution is electrolyzed in a filter press electrolysis cell having a diaphragm and insoluble electrodes. Following electrolysis, the disulfides produced are separated prior to passing the regenerated caustic back to the scrubbing process. The examples teach the use of electrolytic cell current densities of generally less than 0.2 amps per square inch and cell voltages of about 6 volts or the minimum current density required to oxidize the sulfur containing impurities in the spent caustic solution. Just enough oxygen is generated by electrolysis of the spent caustic solution to oxidize all of the mercaptans present to disulfides. The disulfides are physically separated outside the electrolysis cell in a separation vessel. It is apparent that the process that this patent is intended to replace includes the use of oxidation agents, such as tannic acid, and blowing air into the spent caustic solution to oxidize the mercaptans to disulfides. The process would be a more expensive process than the air oxidation step of the prior art but, more importantly, the disclosure omits mention of the fact that during electrolysis, some of the sulfur compounds are oxidized to thiosulfates or sulfate. These oxidized sulfur compounds will eventually build up in the spent caustic electrolyte to the saturation point so as to require either a sulfate removal step or removal by the use of vacuum crystallization or the neutralizing of the spent caustic solution followed by disposal to the environment of the oxidized spent caustic solution. In this process, the disulfides produced during electrolysis of the spent caustic solution are removed to the environment without further oxidation. Under present or contemplated governmental restrictions, disposal of these disulfides may represent an environmental liability, thus requiring destruction prior to discharge to the environment.
In U.S. Pat. No. 6,132,590, a process is taught for the use of an electrolysis cell operating at about 5 to 10 times the current density and at about half the voltage disclosed in U.S. Pat. No. 2,654,706 discussed above. The caustic solution can be regenerated and mercaptans and disulfides can be converted to elemental sulfur, which can be recovered for its chemical value by filtration, and/or converted to an aqueous solution comprising sulfates, which are water soluble and can be disposed of without liability to the environment. In the electrochemical cell, excess oxygen is liberated in the anode compartment of the cell which results in the conversion by direct oxidation of mercaptans, disulfides, and sulfides present as impurities in the spent caustic solution to elemental sulfur and/or sulfates. Rather than vent the excess oxygen to the environment during processing, the excess oxygen can be passed to a column for pre-treatment of the spent caustic solution feedstream. The use of a cationic permselective membrane as a cell separator in the electrolysis cell utilized in the process allows the formation of a pure sodium hydroxide solution in the cathode compartment of the cell with the feeding of deionized water to the cathode compartment. A porous membrane cell separator can be used where recovery of a pure sodium hydroxide solution is not required. Accordingly, by use of the process, it is possible to reduce the purchase of caustic in an amount equivalent to the amount of the sulfur compounds scrubbed from the hydrocarbon process streams and discharged as sulfates. The process is applicable in one embodiment to destroy or to convert to a benign, environmentally non-toxic state, the sulfur-containing compounds or other organic ingredients removed by caustic scrubbing of a hydrocarbon process stream such as those streams from olefin plants, ethane and propane crackers, as well as sour gasoline process streams.
It is therefore an object of the present invention to provide a process and apparatus for treating sulfur-containing spent caustic solutions from refinery operations that produces an environmentally acceptable effluent that can be discharged into the sea or into a conventional sewage treatment system.
Another object of the invention is to provide such a process that is efficient, that requires a minimal capital investment for installation, and has low operating costs.
It is a further object of the invention to provide a process in which the chloride ion present in seawater is converted to a hypochlorite that is reactive with the sulfur-containing species as part of the reaction scheme.